1. Field of the Invention
This invention is related to enclosures for electronic circuits and particularly to ruggedized enclosures for use in installations subjected to hostile environments including destructive shock events and destructive vibration events. In one embodiment, the invention may operate without requiring additional mechanical isolation.
2. Description of the Related Art
Conventional ruggedized electronics enclosures are often employed in military applications. The environments in which military electronic circuits must be able to operate typically present conditions outside of a commercial electronic circuit's operational parameters. Examples of such conditions include excessive moisture, salt, heat, vibrations, and mechanical shock. Historically, military electronic equipment was custom made to provide the required survivability in the hostile environments. While effective in surviving the environment, custom equipment is often significantly more expensive than commercial systems, and is typically difficult if not impossible to upgrade to the latest technologies. Therefore, a current trend in conventional military hardware is to adapt commercially available electronics for use in military applications. These systems are typically known as Commercial Off The Shelf systems, or COTS.
The COTS design philosophy has allowed the military to keep current with technological innovations in computers and electronics, without requiring specialized and dedicated electronic circuit board assemblies. The COTS design methodology is attractive because of the rapidly increasing computational power of commercially available, general-purpose computers. Since the components in a COTS system are commercially available, though usually modified to some extent, the military can maintain an upgrade path similar to that of a commercial PC user. Thus the COTS philosophy allows the military to integrate the most potent electronic components available into their current hardware systems.
While COTS systems have allowed the military to reduce the cost of equipment and to make more frequent upgrades to existing equipment, there are inherent disadvantages to COTS systems. As noted above, military applications must be able to withstand various environmental extremes, including humidity, temperature, shock and vibration. These conditions are typically outside of the operating parameters of commercial electronics and, thus, added precautions and modifications to the physical structures of the equipment must be made to ensure reliability of operation in these environments. Conventional COTS systems typically use two specialized modifications to maintain reliability. These approaches may be used separately, or in combination.
To deploy COTS equipment in hazardous environments, COTS components are housed in a complex ruggedized enclosure or case. One approach, sometimes referred to as “cocooning” places a smaller, isolated equipment rack within a larger, hard mounted enclosure. With this approach shock, vibration and other environmental extremes are attenuated by the isolation system to a level that is compatible with COTS equipment. Another approach, sometimes called Rugged, Off The Shelf (ROTS) seeks to “harden” the COTS equipment, in a manner such as to make it immune to the rigors of the extended environmental conditions to which it is exposed. This later approach strengthens the equipment's enclosure and provides added support for internal components. Both cocooning and ROTS design methodologies must also improve cooling efficiency to accommodate higher operating ambient temperatures. Both approaches suffer from added complexity, size, weight and cost.
The size and complexity exacerbates heat-removal from the enclosure and often complex heat flow routes must be devised in order to maintain a desirable operating temperature. Taken together, these design considerations drastically increase the cost and complexity of such an enclosure.
Commercial systems are typically designed around three main criteria, cost, time-to-market and easy expansion. To deliver on all three design goals, the assumption is that the environment for the system will not be exposed to extreme environmental conditions. Cost is the primary motivator to keeping the packaging simple and inexpensive. The package support structures may have a low cost to keep the system cost from escalating. Keeping costs down to a minimum is counter to the requirements of making a system robust enough to survive a military environment.
To easily accommodate system expansion, computer manufacturers try to simplify the installation of peripheral cards, memory and storage. The idea of having a minimum number of fasteners (i.e., a snap-in-place design) allows the customer easy access and installation of peripherals. The design's modularity preserves the customer's investment. When you couple the commercial constraints with the requirements of the military environment, the design requires a different approach, typically moving the structural changes to the system enclosure and it's attachments. The usual cocooning approach is to design the enclosure to absorb as much of the shock as possible to allow the incumbent system to survive the environment. In practice, this is not easily achieved, especially when using larger, and heavier computer systems. Thus, the idea of completely isolating a commercial system from the rigors of the military environment is difficult to achieve and adds a large cost premium because the rack is the item being modified. The current solution to supporting COTS technology in a military environment described above, adds significant complexity to the system.
Two of the most difficult conditions to design for are vibration and mechanical shock. Mechanical shock and vibration may over time destroy electronic equipment by deforming or fracturing enclosures and internal support structures and by causing electrical connectors, circuit card assemblies and other components to fail. In military applications, as well as in commercial avionics and the automotive industry, electronics must be able to operate while being subjected to constant vibrational forces generated by the vehicle engines, or waves, as well as being subjected to sudden, and often drastic, shocks. Examples of such shocks are those generated by bombs, missiles, depth charges, air pockets, potholes, and other impacts typically encountered by military or commercial vessels. Furthermore, these conditions may also be seen in the operating conditions of a network or telephone server during an earthquake. While providing some protection from shock and vibration, the conventional ruggedized enclosure operating alone cannot provide adequate protection for mission-critical electrical components and circuits.
In order to provide additional protection against shock and vibration, conventional COTS systems mount the ruggedized enclosures described above in a mechanically isolated cocoon. FIG. 1 illustrates a conventional mechanically isolated cocoon. As illustrated in FIG. 1, a cocoon 100 is provided to house the various ruggedized enclosures 110. The cocoon 100 may be attached to a floor 130 and/or a wall 140 of its surroundings. Commonly this includes the fuselage or deck plate of a military vehicle. The cocoon 100 is attached to the surroundings 130, 140 via mechanical isolators 120. A particularly advanced mechanical isolator 120 is the polymer isolator illustrated in FIG. 1, though conventional systems may use any spring-like apparatus to provide the isolation. By attaching the cocoon 100 to its surroundings 130, 140 via mechanical isolators 120, the cocoon 100 is allowed limited movement with five degrees of freedom. This limited movement helps to dampen the effects of shock and vibration.
There are several drawbacks to using the mechanically isolated cocoon 100. In order to reduce the shock to the equipment, the cocoon 100 must be provided with a sway space 150 in which it may move unobstructed. Typically this sway space 150 is four to seven inches in each direction of movement. Thus the cocoon 100 consumes additional space 150 which might otherwise be utilized for other activities or equipment. In military applications, commercial aircraft, as well as automotive applications, space is often at a premium.
Additionally, while the cocoon 100 does isolate the equipment from some vibration and shock, it does not completely isolate the equipment. For example, a conventional cocoon 100 can receive a 60–80 G shock (a “G” is a unit of force equal to the force exerted by Earth's gravity on a body at rest and is used to indicate the force the equipment is subjected to when accelerated by a shock event) and reduce the shock felt by the equipment to 10–15 G's. Typically the performance of the cocoon 100 is limited by sway space available, materials used, and equipment placement within the cocoon 100. Additionally, if the environment around the cocoon 100 moves more than the sway space 150 can accommodate, then the cocoon 100 and its equipment will feel the entire shock event. While a significant reduction in the shock may be experienced, it is important to note that commercial equipment is frequently rated for 5 G's or less. Thus, there is still a significant chance for failure within the system.
To provide the additional shock protection, conventional COTS systems pair the cocoon 100 with the ruggedized enclosures 110, or cocoon. However, while more effective in protecting the equipment from mechanical shock, these ruggedized enclosures 10 work only when the shock isolation system is carefully integrated with the included systems. Since the enclosure is allowed to move, issues such as weight, position, center of gravity and heat removal all have to be balanced. Thus, the cost and complexity of such a system are significantly higher when compared to a commercial system using similar electrical components.
What is needed is a ruggedized enclosure for use in hostile environments which: 1) provides a simplified and effective heat flow design; 2) may utilize COTS components; 3) does not require the use of a mechanical isolator or sway space; 4) provides a high level of shock and vibration protection without need for augmentation; and 5) may be manufactured at low cost.